1. Field of the Invention
This invention is related to the field of head-mounted displays (HMD) and methods of manufacturing such displays.
2. Description of the Related Art
Mixed- and augmented reality (MR-AR) technology is a paradigm where computer-generated digital information is selectively superimposed upon a real-world scene to supplement a user's sensory perception of the physical environment. It has been explored for a wide range of applications for 3D scientific visualization, medical training, and engineering processes. One of the enabling technologies in MR-AR systems is a 3D display that is able to seamlessly combine virtual and real information, which is called creating “see-through” capability.
Optical see-though head-mounted displays (OST-HMD) have been one of the basic approaches to combining computer-generated virtual objects with the views of real-world scenes required for MR-AR systems. In an OST-HMD, the direct view of the physical world is maintained and computer-generated virtual images are optically superimposed onto the real scene via an optical combiner. This optical approach allows a user to see the real world in full resolution and introduces less intrusion into the view of the real world than video see-through displays where real-world views are captured through video cameras. Therefore, an OST-HMD system is a system suitable for tasks where eye-hand coordination or non-blocked real-world view is critical.
Designing a wide field-of-view (FOV), compact and non-intrusive OST-HMD, has been a challenge. One head-mounted projection display (HMPD) that deviates from the conventional approaches to HMD designs is illustrated in FIG. 1 as a monocular HMPD configuration. FIG. 1 is a schematic illustration of a monocular head-mounted projection display 10 receiving an image to be displayed from an image source 12. Two major aspects distinguish the HMPD technology from conventional HMDs and projection systems: 1) projection optics 14 replace an eyepiece- or microscope-type lens system in the conventional HMD design, and 2) a retroreflective screen 16 substitutes for a typical diffusing screen in the conventional projection system. The projected light 18 is thus directly retroreflected back to the exit pupil 20 of the projection display 10 where the eye is positioned to view the projected image 22 through beam splitter 24. This combination of projection and retroreflection not only enables stereoscopic capability but also provides intrinsically correct occlusion of computer-generated objects by real ones and offers the capability of designing wide FOV, low distortion optical see-through displays.
However, the images appearing on optical see-through displays commonly lack brightness and contrast compared to the direct view of a real-world scene. While the luminance level of an immersive HMD is usually required to be equal to or greater than about 17 cd/m2 for optimal visual acuity, the image brightness of an OST-HMD should match the average luminance level of its working environments. The average luminance of outdoor scenes is typically about 5000 to 6000 cd/m2, and a well-lit indoor environment approximately averages 400˜500 cd/m2. State-of-the-art microdisplays suitable for HMDs yield 100 cd/m2 of luminance on average for backlit active-matrix liquid crystal displays (AMLCD), 300 to 1000 cd/m2 for liquid crystal on silicon (LCOS) displays, and 50 to 600 cd/m2 for organic light emitting displays (OLED).
The problem of low image brightness and contrast is worsened by any light attenuation through any optical combiner interface required in see-through displays, resulting in low luminance transfer efficiency of the optical system. In conventional OST-HMDs, a 50/50 beamsplitter will attenuate the light, from both a displayed image and the real scene, by 50%. Consequently, such displays are usually used in dim environments, reducing the feasibility of applying such information displays outdoor or in scenarios where well-lit environments such as in an operation room are necessary.
The low-efficiency problem is aggravated in a see-through HMPD in which the projected light is split twice through beam splitter 24 as illustrated in FIG. 1. Using a 50/50 beam splitter leads to the loss of at least 75% of the light from a displayed image and 50% of the light from the real scene. The light from the displayed image is further attenuated by as high as 80% through an imperfect retroreflective screen. The actual luminance returned back to the exit pupil is around 4˜10% or less of the display luminance. For instance, providing the usage of AMLCDs, the observed peak luminance is about 4 to 10 cd/m2 or lower. This luminance imposes significant restrictions on the lighting conditions of working environments and limits applications demanding well-lit environments.
The following references, whose contents in entirety are incorporated herein by reference, represent background techniques and procedures used conventionally for head-mounted displays:                1. J. P. Rolland, and H. Fuchs, “Optical versus video see-through head-mounted displays in medical visualization,” Presence: Teleoperators and Virtual Environments (MIT Press), 9(3), 287-309, 2000.        2. R. Fisher, “Head-mounted projection display system featuring beam splitter and method of making same,” U.S. Pat. No. 5,572,229, 1996.        3. J. Fergason. “Optical system for head mounted display using retro-reflector and method of displaying an image”, U.S. Pat. No. 5,621,572. Apr. 15, 1997.        4. H. Hua, A. Girardot, C. Gao, and J. P. Rolland “Engineering of head-mounted projective displays”. Applied Optics, 39 (22), pp. 3814-3824, 2000.        5. H. Hua, C. Gao, and J. P. Rolland, “Study of the imaging properties of retro-reflective materials used in head-mounted projective displays (HMPDs),” in Aerosense 2002, April 1-5 th, Orlando, Fla.        6. H. Hua and C. Gao, “A polarized head-mounted projective displays,” in Proc. of IEEE and ACM International Symposium on Mixed and Augmented Reality 2005, pp. 32-35, 2005.        7. R. Kijima and T. Ojika, “Transition between virtual environment and workstation environment with projective head-mounted display”, Proc. of IEEE VR 1997, pp. 130-137, 1997.        8. J. Parsons and J. P. Rolland, “A non-intrusive display technique for providing real-time data within a surgeons critical area of interest,” Proc. of Medicine Meets Virtual Reality 1998, 246-251, 1998.        9. N. Kawakami, M. Inami, D. Sekiguchi, Y. Yangagida, T. Maeda, and S. Tachi, “Object-oriented displays: a new type of display systems—from immersive display to object-oriented displays”, Proc. of IEEE SMC 1999, IEEE International Conference on Systems, Man, and Cybernetics, Vol. 5, pp. 1066-9, 1999.        10. M. Inami, N. Kawakami, D. Sekiguchi, Y. Yanagida, T. Maeda, and S. Tachi, “Visuo-haptic display using head-mounted projector”, Proc. IEEE Virtual Reality 2000, pp. 233-40, 2000.        11. D. Poizat and J. P. Rolland, “Use of retro-reflective sheets in optical system design,” Technical report TR98-006, University of Central Florida, Orlando, Fla., 1998.        12. H. Hua, Y. Ha, and J. P. Rolland, “Design of an ultra-light and compact projection lens,” Applied Optics, 42(1), 1-12, 2003.        13. H. Hua, C. Gao, F. Biocca, and J. P. Rolland, “An Ultra-light and Compact Design and Implementation of Head-Mounted Projective Displays,” Proc. of IEEE VR 2001, pp. 175-182, 2001.        14. Y. Ha, Hong Hua, R. Martins, and J. P. Rolland, “Design of a wearable wide-angle projection color display,” in Proc. of International Optical Design Conference 2002 (IODC), 2002.        15. J. P. Rolland, F. Biocca, F. Hamza-Lup, Y. Ha, and R. Martins, “Development of head-mounted projection displays for distributed, collaborative, augmented reality applications,” Presence: Teleoperators and Virtual Environments, 14(5), 528-549, 2005.        16. R. Martins, V. Shaoulov, Y. Ha, and J. P. Rolland, “Projection-based head-mounted displays for wearable computers,” Proc. of SPIE, Vol. 5442, pp. 104-110, 2004.        17. C. Curatu, H. Hua, and J. P. Rolland, “Projection-based head-mounted display with eye-tracking capabilities,” Proc. of SPIE, Vol. 5875, 2005.        18. M. Inami, N. Kawakami, and S. Tachi, “Optical camouflage using retro-reflective projection technology,” Proc. of ISMAR 2003, pp. 348-349, 2003.        19. H. Hua, L. Brown, & C. Gao, “System and interface framework for SCAPE as a collaborative infrastructure,” Presence. Teleoperators and Virtual Environments, 13(2), 234-250, April 2004.        